Caution urged as estuary kicks up a stink
People are being asked to avoid waterbased activities at the New River Estuary because of an unpleasant smell.
Environment Southland coastal scientist Nick Ward said the council was investigating the cause.
‘‘It is something we’ve been investigating, but it’s a work in progress,’’ he said.
‘‘In the meantime, it would be wise for people to stay away from areas where it’s smelly and also where there’s black sediment.’’
The estuary had experienced excessive nutrient and sediment input from waterways for several years, he said.
This shift in nutrient levels created a growth bonanza for red and green algae and ‘‘not-so-great’’ conditions for important fish habitat species such as seagrass, he said.
‘‘This prolific growth results in lots of organic matter, which rots down. The bacteria that causes it to rot eventually uses up the oxygen and, when that runs out, it uses other sources to get the energy it needs, such as iron, nitrate,
and eventually carbon phate.’’
In the sulphate reduction stage the sulphur loses oxygen to form hydrogen sulphide, he said. ‘‘Hydrogen sulphide smells like rotten eggs; it’s the same smell as you get in Rotorua,’’ he said.
‘‘Due to the low river levels, warm conditions and lower tide levels, more
dioxide,
sul- sediment is exposed and the gas it produces is more able to disperse, which is why people are picking up on the smell.’’
The council monitors bathing water from early October until the end of March. Results are available online, but conditions can change daily and people are advised to be cautious.